In an Orthogonal Frequency Division Multiplexing (OFDM) modulation system employed in a DVB-T transmitter and receiver system, the channel state information (CSI) is applied to increase the reliability of each sub-carrier so that the Bit Error Rate (BER) after a channel decoder can be decreased. It is significantly important in a frequency selective fading channel, and methods generating CSI have been disclosed. Generally, these channel state informations are defined based on a magnitude of a channel frequency response signal in each sub-carrier or an SNR (signal to noise ratio) of each sub-carrier. The former method can have good performance when the noise brought to the channel is Additive White Gaussian Noise (AWGN) or in a static channel. However, it does not exhibit good performance in a channel with co-channel interference, which is the real environments where the OFDM signals are broadcasted. For example, some inherent analog TV channel may be mixed with the DVB-T (Digital Video Broadcasting Terrestrial) channel and co-channel interference is generated as a result. In such case, using magnitude of a channel frequency response may result in an unreliable CSI caused by severe interference to those sub-carriers. The method using a temporal-averaged SNR of each sub-carrier as CSI can solve this problem of co-channel interference. However, there are two drawbacks of this method. First, an inaccuracy of SNR estimation may cause additional degradation in AWGN environment. Second, a temporal-averaged SNR does not respond quickly to the channel change in mobile environment.
Please refer to FIG. 1, which shows a portion of a transmitted DVB-T frame including the scattered pilot carriers and data carriers transmitted in the DVB-T system. The transmitter in a DVB-T system may generate the I (In-phase) and the Q (Quadrature) data signals according to a modulation format, such as QPSK (Quadrature Phase Shift Keying), 16-QAM (Quadrature Amplitude Modulation), 64-QAM, etc., the pilot signals and TPS signals, corresponding to the BPSK constellation, are inserted according to the DVB-T frame structure, and the I/Q signals of all sub-carriers are transformed into a time domain OFDM symbol. Each symbol may be made up of a number of active carriers, and the number of active carriers is depending on the operation mode. For example, there may be 6817 active carriers in 8K mode, 3409 active carriers in 4K mode or 1705 active carriers in 2K mode, as shown in FIG. 1. In this drawing, the scattered pilot carriers are marked with solid circles, and data carriers are marked with empty circles. The aforementioned rules relating to modulation, number of active carriers, and operation mode are described in European Telecommunication Standard Institute standards.
Please refer to FIG. 2, a conventional CSI calculating method using magnitude of a channel frequency response signal in each sub-carrier is shown, where c(k) is the magnitude of a channel frequency response and “k” is the index representing each sub-carrier. The conventional method directly squares the magnitude of the channel response signal as the CSI for decoding the OFDM symbol subsequently. This method is based on an assumption that the noises in all sub-carriers are flat, (i.e. the noise in each sub-carrier is of equal power). Therefore, the sub-carrier with larger channel frequency response through such calculation results a larger CSI. However, an unpredictable co-channel interference comes as an unavoidable consequence of channel reuse, which makes the assumption of flat noise incorrect. In addition, the amount of co-channel interference is determined by how the channels are reused spatially. Therefore, a method correctly estimating the CSI of each sub-carrier is required in a real environment.
Please refer to FIG. 3, another conventional CSI calculating method using the time-averaged SNR of each sub-carrier determining CSI is shown. This method can solve the aforementioned problem resulted from the co-channel interference. The e(k) is the error signal, and “k” is the index representing each sub-carrier. As shown in FIG. 6, the error signal e(k) is defined as the minimum magnitude of the difference from the equalized result to the ideal constellation point. The mean square error (MSE) of the error signals is calculated directly by temporarily averaging (recursive averaging over OFDM symbols by the Infinite Impulse Response Filters, IIR average unit 302) the square of error signals and the MSE is inversed to obtain the CSI for decoding the OFDM symbol later. This method can mitigate the co-channel interference or other types of interference caused by other reasons, e.g. interference caused by adjacent channels. However, the drawback of this method is the bad performance in mobile channel. In a mobile channel environment, the SNR in a sub-carrier can change with time; the conventional method derives only the averaged SNR and does not take the time variation of the channel into consideration.
Either of the conventional methods for generating CSI as described with reference to FIG. 2 or FIG. 3, extracts the scattered pilot carriers and calculates the I/Q signals of each sub-carrier to generate CSI as well known. The CSI of kth carrier is indicated by csi(k), which is applied when generating the soft bits for the channel decoder. As described above, the conventional method shown in FIG. 2 employs the magnitude of the channel response signal but the conventional method shown in FIG. 3 employs the temporal-averaged SNR to generate the CSI extracted from each sub-carrier of the OFDM symbols. Accordingly, either of the conventional methods exhibits good performance only in particular environments, such as an AWGN environment (white noise) or static channel with co-channel interference (color noise). Considering a real environment, there is a need to develop a method and apparatus exhibiting good performance to work well in either a static or mobile channel with either AWGN or co-channel interference.